From 90309c01a811c7e05baf6d95a2fde2bdd4cdeaf5 Mon Sep 17 00:00:00 2001
From: athenian200 <athenian200@outlook.com>
Date: Sun, 25 Oct 2020 14:27:17 -0500
Subject: Issue #1673 - Part 3: Bring minimum tab advance up to spec.

This provides a clearer rule for the minimum tab advance that brings us to alignment with the spec and both major browsers.
---
 layout/generic/nsTextFrame.cpp | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index 83e50ca430..a93c71b289 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -1718,6 +1718,16 @@ GetSpaceWidthAppUnits(const gfxTextRun* aTextRun)
   return spaceWidthAppUnits;
 }
 
+static gfxFloat
+GetMinTabAdvanceAppUnits(const gfxTextRun* aTextRun)
+{
+  gfxFloat chWidthAppUnits =
+    NS_round(GetFirstFontMetrics(aTextRun->GetFontGroup(),
+			         aTextRun->IsVertical()).zeroOrAveCharWidth *
+	     aTextRun->GetAppUnitsPerDevUnit());
+  return 0.5 * chWidthAppUnits;
+}
+
 static nscoord
 LetterSpacing(nsIFrame* aFrame, const nsStyleText* aStyleText = nullptr)
 {
@@ -3092,6 +3102,7 @@ public:
       mLength(aLength),
       mWordSpacing(WordSpacing(aFrame, mTextRun, aTextStyle)),
       mLetterSpacing(LetterSpacing(aFrame, aTextStyle)),
+      mMinTabAdvance(-1.0),
       mHyphenWidth(-1),
       mOffsetFromBlockOriginForTabs(aOffsetFromBlockOriginForTabs),
       mReflowing(true),
@@ -3116,6 +3127,7 @@ public:
       mLength(aFrame->GetContentLength()),
       mWordSpacing(WordSpacing(aFrame, mTextRun)),
       mLetterSpacing(LetterSpacing(aFrame)),
+      mMinTabAdvance(-1.0),
       mHyphenWidth(-1),
       mOffsetFromBlockOriginForTabs(0),
       mReflowing(false),
@@ -3180,6 +3192,13 @@ public:
 
   void CalcTabWidths(Range aTransformedRange, gfxFloat aTabWidth);
 
+  gfxFloat MinTabAdvance() {
+    if (mMinTabAdvance < 0.0) {
+      mMinTabAdvance = GetMinTabAdvanceAppUnits(mTextRun);
+    }
+    return mMinTabAdvance;
+  }
+
   const gfxSkipCharsIterator& GetEndHint() { return mTempIterator; }
 
 protected:
@@ -3212,6 +3231,7 @@ protected:
   int32_t               mLength; // DOM string length, may be INT32_MAX
   gfxFloat              mWordSpacing;     // space for each whitespace char
   gfxFloat              mLetterSpacing;   // space for each letter
+  gfxFloat		mMinTabAdvance; // min advance for <tab> char
   gfxFloat              mHyphenWidth;
   gfxFloat              mOffsetFromBlockOriginForTabs;
 
@@ -3467,14 +3487,13 @@ PropertyProvider::GetSpacingInternal(Range aRange, Spacing* aSpacing,
 
 // aX and the result are in whole appunits.
 static gfxFloat
-AdvanceToNextTab(gfxFloat aX, nsIFrame* aFrame,
-                 gfxTextRun* aTextRun, gfxFloat aTabWidth)
+AdvanceToNextTab(gfxFloat aX, nsIFrame* aFrame, gfxTextRun* aTextRun,
+	         gfxFloat aTabWidth, gfxFloat aMinAdvance)
 {
 
-  // Advance aX to the next multiple of *aCachedTabWidth. We must advance
-  // by at least 1 appunit.
-  // XXX should we make this 1 CSS pixel?
-  return NS_round((aX + 1) / aTabWidth) * aTabWidth;
+  // Advance aX to the next multiple of aTabWidth. We must advance
+  // by at least aMinAdvance.
+  return ceil((aX + aMinAdvance) / aTabWidth) * aTabWidth;
 }
 
 void
@@ -3537,7 +3556,7 @@ PropertyProvider::CalcTabWidths(Range aRange, gfxFloat aTabWidth)
           mFrame->SetProperty(TabWidthProperty(), mTabWidths);
         }
         double nextTab = AdvanceToNextTab(mOffsetFromBlockOriginForTabs,
-                mFrame, mTextRun, aTabWidth);
+	        mFrame, mTextRun, aTabWidth, MinTabAdvance());
         mTabWidths->mWidths.AppendElement(TabWidth(i - startOffset, 
                 NSToIntRound(nextTab - mOffsetFromBlockOriginForTabs)));
         mOffsetFromBlockOriginForTabs = nextTab;
@@ -8335,8 +8354,8 @@ nsTextFrame::AddInlineMinISizeForFlow(nsRenderingContext *aRenderingContext,
 	tabWidth = ComputeTabWidthAppUnits(this, textRun);
       }
       gfxFloat afterTab =
-        AdvanceToNextTab(aData->mCurrentLine, this,
-                         textRun, tabWidth);
+        AdvanceToNextTab(aData->mCurrentLine, this, textRun, tabWidth,
+			 provider.MinTabAdvance());
       aData->mCurrentLine = nscoord(afterTab + spacing.mAfter);
       wordStart = i + 1;
     } else if (i < flowEndInTextRun ||
@@ -8497,8 +8516,8 @@ nsTextFrame::AddInlinePrefISizeForFlow(nsRenderingContext *aRenderingContext,
 	tabWidth = ComputeTabWidthAppUnits(this, textRun);
       }
       gfxFloat afterTab =
-        AdvanceToNextTab(aData->mCurrentLine, this,
-                         textRun, tabWidth);
+        AdvanceToNextTab(aData->mCurrentLine, this, textRun, tabWidth,
+			 provider.MinTabAdvance());
       aData->mCurrentLine = nscoord(afterTab + spacing.mAfter);
       lineStart = i + 1;
     } else if (preformattedNewline) {
-- 
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